1. Field of the Invention
The present invention relates to a laser probe which is to be attached to the tip end of a laser handpiece grasped by an operator, and which is made contact with a diseased part of an organic hard tissue such as a bone so as to irradiate the diseased part with laser light.
2. Description of the Related Art
In laser treatment performed in various medical fields, a diseased part is irradiated with laser light in order to carry out vaporization, dissection, coagulation, or hemostatis of an organic soft tissue such as epithelium, musculus, or nervus, and vaporization of an organic hard tissue such as a bone or a tooth bud. For example, an Nd:YAG solid-state laser is used for the purpose of coagulation and hemostatis of the organic soft tissue. A laser light is condensed at the tip end of a conical laser probe made of quartz or sapphire, and the organic soft tissue is irradiated with the laser light to perform treatment such as vaporization, dissection, coagulation, or hemostatis.
As a laser treatment apparatus for treatment of the organic hard tissue, a laser treatment apparatus using an Er:YAG solid-state laser of a wavelength of 2.94 .mu.m is known. In the apparatus, a laser probe comprising an optical fiber is detachably attached to the tip end of a handpiece, and laser light guided to the handpiece is emitted from the optical fiber of the laser probe. According to the apparatus, treatment is performed by irradiating a diseased part of the organic hard tissue with the laser light so as to vapor the diseased part of the organic hard tissue.
Generally, an optical fiber has a three-layer structure consisting of a core, a clad, and a jacket. The core which is at the center has a refractive index which is higher than that of the clad covering the core. The jacket covers the core and the clad so as to protect them. In accordance with the refractive index distribution in a radial direction of a section of the core, optical fibers are roughly classified into two kinds; a step index optical fiber in which the refractive index is uniform in the refractive index distribution along the direction; and a graded index optical fiber in which the refractive index is gradually reduced as moving from the center of the core to the clad along the direction. In the step index optical fiber, light is propagated with being totally reflected at the interface between the core and the clad. By contrast, in the graded index optical fiber, light is propagated while meandering in the core. A energy density in the irradiation plane of laser light emitted from the light emitting end of such the step index optical fiber is often distributed in accordance with a Gaussian distribution centered at the vicinity of an extension line of the center axis of the optical fiber. In other words, the energy density is higher in the vicinity of an extension of the center axis of the optical fiber, and lower in the vicinity of an extension of the interface between the core and the clad.
In a prior art laser probe used in the above-mentioned laser treatment apparatus for the organic hard tissue, such the step index optical fiber is mainly used. In the laser probe using such the step index optical fiber, when laser light in the form of continuous pulses is emitted for about 5 minutes, the light emitting face is damaged and peeled off, with the result that the vaporization efficiency of the organic hard tissue is lowered. Owing to such a damage or peeling off, the durability of the probe is very low. It is considered that this phenomenon takes place because of the following reason.
In such the laser treatment apparatus, laser light is guided from a laser light source to the body of the handpiece via a guide optical fiber of laser light guiding means, and then enters the optical fiber of the laser probe having a center optical axis which coincides with that of the guide optical fiber. In the guide optical fiber of the laser light guiding means, for example, the laser light is guided in such a manner that the energy level becomes higher as moving toward the center axis of the fiber. The optical fiber of the prior art laser probe to which laser light is guided from such the guide optical fiber has no function of changing the energy distribution of laser light so as to be uniform. When laser light enters as it is the optical fiber of the laser probe, therefore, the energy density of the laser light at the center of the core in the light emitting face of the optical fiber is larger than that in the periphery of the core. This causes the center of the core in the light emitting face to be peeled off or damaged, thereby impairing the durability of the probe.
In the above-described laser treatment apparatus for the organic hard tissue, when the apparatus is to be used for evaporating the organic hard tissue, a laser probe which uses the conventional step optical fiber is mainly used as described above, and the laser probe is attached to the tip end of the handpiece body. The laser probe is made contact with the organic hard tissue, and then the tissue is irradiated with laser light. At this time, organic components of the evaporated organic hard tissue, such as phosphorus, sulfur, and calcium are deposited on the light emitting face of the optical fiber of the laser probe.
When such organic components are deposited on the light emitting face of the optical fiber of the laser probe, heat is generated by laser light absorbed by the deposition and the heat elevates the temperature of the light emitting face. At the same time, the deposition of such organic components causes the melting point of quartz glass (SiO.sub.2) which is a principal material of the core, to be lowered. As a result, the light emitting end of the optical fiber of the laser probe is melted at a temperature lower than the melting point of quartz glass, and the light emitting face of the laser probe is peeled off or damaged. This damage proceeds vigorously, particularly in the vicinity of the center axis of the core where the energy density is high. When the organic hard tissue is treated by using such a laser treatment apparatus as described above, furthermore, water is usually supplied in a mist form to a diseased part. Therefore, a thermal stress is produced in the light emitting face of the optical fiber of the laser probe, thereby causing the light emitting face to be further easily peeled off or damaged.
FIG. 17 is a section view showing an optical fiber 1 of a laser probe of the prior art, and more specifically a section view showing the state of the light emitting end of the optical fiber 1 obtained after the laser probe is attached to the above-mentioned laser treatment apparatus of the prior art and the organic hard tissue is continuously irradiated for 20 to 30 seconds with laser light pulses to be vapored. For the sake of convenience in description, a jacket is not illustrated. The optical fiber 1 comprises a core 4 and a clad 5. When the optical fiber 1 is attached to the laser treatment apparatus having the laser probe, and the organic hard tissue is irradiated with continuous pulses of laser light to be vapored, as shown in FIG. 17, for example, the core 4 is cut away after a lapse of 20 to 30 seconds, from the light emitting face 6 before the use and indicated by the two-dot chain line, and the current light emitting face 2 is retracted. When the core 4 is cut away in this manner, the light emitting face 2 has a concave shape or is not flat, so that laser light is scattered by the light emitting face 2, and a gap is formed between the light emitting face and a diseased part. As a result, the vaporization amount of the organic hard tissue per unit time is largely reduced and it is impossible to treat the diseased part.
When continuous pulses are applied via the optical fiber 1 of the laser probe for 20 to 30 seconds so as to vapor the organic hard tissue, for example, the vaporization ability is lowered up to about one-third on average. Therefore, it is difficult to continuously conduct vaporization while uniformly maintaining the vaporization efficiency. In order to maintain the vaporization ability in the laser treatment apparatus, the laser probe comprising the optical fiber 1 must be frequently replaced with a fresh one.
An irradiation apparatus which converts laser light emitted from a laser generator so as to have a laser beam intensity with a Gaussian distribution, into laser light of uniform laser beam intensity, and transmits and applies the converted laser light is disclosed in Japanese Unexamined Patent Publication JP-A 2-297986 (1990). In the irradiation generator, a polygon prism is interposed between the laser device and a guide optical fiber through which laser light is transmitted. When laser light with a Gaussian distribution in both vertical and lateral directions passes through the polygon prism, for example, the intensity distribution of the laser light is uniformized. In the disclosed irradiation apparatus, the strength of laser light emitted from the laser device can be uniformized, but it is difficult to prevent nonuniformity in the laser intensity distribution due to different refractive indices in the optical fiber or the like from occurring in the light emitting end of the optical fiber of the laser probe.
As another prior art technique there is known an ophthalmological laser treatment apparatus which irradiates a diseased part with laser light by means of a laser probe using an optical fiber. The laser treatment apparatus employs pulsate argon laser of a wavelength of 0.514 .mu.m in order to perform intraocular photocoagulation of a retina. When treatment is performed by laser irradiation from the apparatus, the face to be irradiated can be irradiated with laser light from the light emitting end of the optical fiber of the laser probe, with a uniform energy density. Therefore, excessive or insufficient coagulation hardly occurs.
In the laser treatment apparatus described above, the eye which is an object to be irradiated with laser light is an organic soft tissue. In laser light for intraocular photocoagulation, therefore, a peak value of a pulse (a height of the pulse) which indicates an irradiation energy level is low or about one-thousandth of that of laser light for vaporization of the organic hard tissue. When such low level laser light is emitted via a laser probe comprising the above-described optical fiber of the prior art, the light emitting face of the optical fiber of the laser probe is hardly affected by the laser light because the energy level of the laser light is low, and hence the light emitting face of the optical fiber is not damaged. By contrast, as described above, the durability of a laser probe for emitting high power level laser light which is used for vaporing the organic hard tissue is low because of a damage of the light emitting face of the optical fiber. Consequently, it has been requested to develop a laser probe for organic hard tissues in which the light emitting face of an optical fiber is not damaged and hence the durability is improved.